This invention relates to a device for processing thermally developable papers and films. More particularly, it relates to an apparatus which allows removal of evaporated moisture from thermally developable papers during thermal development so as to bring about uniform development.
A typical thermally developable paper or film comprises a base layer having a coating thereon of a photothermographic layer which comprises a binder, a photo-sensitive silver halide which photogenerates silver, a reducing agent for silver ions, and a reducible light-insensitive silver source (e.g., a salt), the reduction of which source is catalyzed by silver. The developed image is formed in the thermographic layer.
The development of a latent image in the photothermographic layer of a heat developable paper or film requires that the layer be maintained at a temperature within its development temperature range for a predetermined period of time. The period is shorter at higher development temperatures. One of the more difficult problems encountered in heating a thermally developable sheet of paper or film uniformly to create an image with no irregularities has been the presence of moisture in the sheet. When a sheet is heated, as by contact with a heating surface, moisture within the sheet creates pockets of steam between the sheet and the heating surface, causing non-uniform heat transfer and uneven mid-tone gray levels. When the moisture level of a paper sheet is very high, wrinkling of the sheet often occurs during development. In some cases, portions of the sheet are insufficiently heated and are not totally processed. Uniformity of heat transfer is critical. Even though a thermal developer apparatus may have adequate temperature control on the heating surface, the temperature experienced by the paper or film recording medium will be uniform only with uniform contact between the heating surface and recording medium.
Conventional thermal processing devices can be characterized as one of three types:
A. Rigid or semi-rigid hot shoe heating member and drive roller transport member; PA1 B. Rigid platen heating member and belt drive transport member; and PA1 C. Rigid drum heating member and paper/film supply roll transport member. PA1 1. The first is a semi-rigid shoe to which heat is supplied by means of a heat source such as a heating blanket bonded to the back side of the hot shoe; PA1 2. The second is a smaller extruded rigid shoe with a cartridge heater of the type used in many microfilm and copying machines; PA1 3. The third is a larger extruded rigid shoe with grooves formed therein to alleviate the moisture problem. The grooves are not open slots or holes in the shoe and do not easily allow moisture vapor to pass from the imageable sheet to the atmosphere during development. PA1 (a) means for heating the papers and films, which means define a heating zone, PA1 (b) means within said heating zone for allowing moisture from the papers and films to be vented to the atmosphere, and PA1 (c) means for applying pressure uniformly between the heating means and the papers and films.
A hot shoe may be defined as a heated path or course that positions or otherwise influences, as by friction, the movement of a strip of exposed photothermographic paper or film in its passage through a development zone. A typical hot shoe can be described as a heated sleeve-like member, said member being semi-cylindrical in shape. The exposed photothermographic paper or film is contacted with the concave side of the semi-cylindrical sleeve-like member. A drive roller, a rotating cylinder which conforms to the shape of the concave side of the sleeve-like member, drives the paper or film over the heated path by means of a frictional driving force. The drive roller thus transports the paper or film through the heating region, allowing the paper or film to be heated by the sleeve-like member while providing pressure to keep the paper or film against the concave side of the member. In the situation described, the path or course which positions the strip of exposed paper or film is the area defined by the concave side of the sleeve-like member. Three basic hot shoe/roller developer devices exist:
The foregoing hot shoe/drive roller devices have no provision to allow moisture to escape from the heating zone. The entrapped moisture, combined with non-uniformities in pressure resulting from tolerance and concentricity imperfections between the hot shoe and drive roller, result in uneven processing, particularly under moisture conditions wherein the amount of moisture in the paper exceeds about 3.5 percent by weight.